Apple will provide software and hardware support for any given product for at least 5 years. After those 5 years, you sometimes will still get security fixes.

The reason for this is that newer software will start using hardware features and capabilities that only exist on newer hardware, not because Tim Cook is evilly cackling in his office "hahhahha! Let's force people to buy new Macs!!!"

Erm, isn't the last bit a key part of Tim Cooks job (getting people to buy new apple stuff even if they don't really need it)?